Absorbance detection system for lab-on-a-chip

ABSTRACT

A high-efficiency, high-sensitivity absorbance detection system in a lab-on-a-chip is provided. The absorbance detection system includes detection cells having an optical pathlength ten times and/or much longer than the width of a separation channel to improve detection sensitivity, lens structures for collimating light in the detection cells, and slit structures for preventing scattered light from entering detectors. The detection cells, the lens structures, and the slit structures of the absorbance detection system are fabricated and integrated in a lab-on-a-chip. The absorbance detection system exhibits excellent absorption efficiency, detection limit, and linearity, compared to existing absorbance detection systems, and can be applied for the detection of a variety of samples. The absorbance detection system does not need labeling of the samples which saves time and costs. The absorbance detection system can be used effectively in detecting trace compounds with a high sensitivity. The absorbance detection system in a lab-on-a-chip can be used with wider applications to a variety of samples in diverse research fields, such as the drug screening field dealing with simultaneous synthesis and identification of a number of compounds based on combinatorial chemistry, the life sciences field handling trace bioactive materials such as enzymes, proteins, and amino acids, and the environmental research field which needs rapid field monitoring of contaminants.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an absorbance detection systemin a lab-on-a-chip, and more particularly, to a high-efficiency,high-sensitivity absorbance detection system in which detection cellswith long optical path lengths for higher detection sensitivity,micro-lenses for collimating light into the detection cells, and slitsfor preventing scattered light from entering detectors are fabricatedand integrated in a lab-on-a-chip.

[0003] The present invention of an absorbance detection system in alab-on-a-chip is capable of solving the problems of conventionalabsorbance detection systems in lab-on-a-chips and produces a 10 timesgreater detection sensitivity.

[0004] The invented detection system can utilized a liquid- orsolid-state waveguides or micro-light sources (lamp or laser) as well asoptical fibers for light radiation.

[0005] The present invention has additional collimators includingmicro-lenses and slits arranged close to detection cells so thateffective absorbance detection can be achieved using the detection cellswith the 50 μm or greater optical pathlengths.

[0006] The present invention of an absorbance detection system in alab-on-a-chip has collimators including a micro-lenses for collimatingdivergent light from optical fibers and transmitting the collimatedlight to detection cells. Slits are built-in to prevent light that doesnot pass through the detection cell or scattered light from enteringdetectors so that detection sensitivity can be improved markedly withincreased optical pathlength of the detection cell.

[0007] 2. Description of the Related Art

[0008] A variety of analytical instruments such as capillaryelectrophoresis (CE), liquid chromatography, and gas chromatography areused to separate and analyze mixed compounds. In particular, CE andliquid chromatography have wide applications in conjunction with avariety of available detection methods, such as absorbance detection,fluorescence detection, electrochemical detection and others. Thefluorescence detection method advantageously has a high detectionsensitivity but needs fluorescence labels to be coupled to samplesbecause self-luminescent substances rarely exist. The electrochemicalmethod also has a high detection sensitivity but its application islimited to specific compounds. Whereas, the absorbance detection methodcan be applied to a wide range of analytes and does not need labeling ofanalytes for detection, and thus it has been the most popular detectionmethod.

[0009] In the absorbance detection method based on Beer's law, theabsorbance of a sample is proportional to the distance light passesthrough the sample, i.e., the optical pathlength, which is expressed by:

A=ε×b×C  (1)

[0010] where A is absorbance, ε is the molar extinction coefficient(L/mol·cm), b is the optical pathlength (cm), C is the molarconcentration of the sample (mol/L).

[0011] Sensitivity of the absorption detection is usually poor in CE.This is because the capillary used for CE has a small inner diameter of50-100 μm, and thus the optical pathlength is very short. In addition,because the capillary has a circular cross-section, a portion of lightpasses through the capillary, and thus the actual pathlength is smallerthan the inner diameter of the capillary.

[0012] In an attempt to increase the sensitivity of absorbance detectionin CE, detection cells with an extended optical pathlength have beendeveloped. Typically, the use of a capillary having a rectangularcross-section or a U-shaped or Z-shaped detection cell has beensuggested to increase the optical pathlength by 10-50 times the innerdiameter of a common capillary.

[0013] In a lab-on-a-chip based CE system, the depth of micro-channelsformed in a glass plate or plastic plate is as small as 10-30 μm, andthus the absorbance detection sensitivity in the microchip system isworse than that in CE. For this reason, an attempt to increase detectionsensitivity by applying a U-shaped detection cell in a lab-on-a-chipsystem has been made. In this approach, optical fibers are arranged infront of and behind the detection cell, and light is radiated into thedetection cell through an optical fiber and collected by another opticalfiber for detection.

[0014] Lab-on-a-chip systems for chemical/biological analysis will bedescribed briefly. Lab-on-a-chip systems fabricated by a micro-machiningtechnique such as a photolithography technique used in the manufactureof semiconductor devices are referred to as chemical microprocessorsincluding a variety of components (for sample pretreatment, injection,reaction, separation and detection) integrated in a glass, silicon, orplastic substrate of an area of several square centimeters. Theselab-on-a-chip systems advantageously enable high-speed, high-efficiency,high-cost automated chemical/biological analysis to be carried out juston the one device.

[0015] In most lab-on-a-chip based analytical systems, migration andseparation of a sample are performed by electroosmotic flow induced bythe application of voltages to both ends of a microchannel filled with asample solution. The microfluidics in a microchip can be controlled byapplying high voltages and thus eliminating the use of a mechanical pumpor valve. This has enabled the microchip to be fabricated to muchsmaller sizes than other commercially available analytical systems andat relatively low costs. In addition, a series of sample injection,migration, reaction, separation and detection processes can be performedcontinuously in a single lab-on-a-chip.

[0016] Although the lab-on-a-chip-based analytical systems describedabove are advantageous in that the consumption of sample and reagents isreduced and the analysis can be performed within a short period of time,they cannot be applied to the analysis of a variety of samples due tolimited detection methods. So, fluorescence detection andelectrochemical detection methods are commonly used for detection in alab-on-a-chip. To compensate for the drawback of the lab-on-a-chip-basedanalytical systems and to extend its applications, a glass basedlab-on-a-chip integrated with an absorption detection system usingoptical fibers was developed. In this system, a single mode opticalfiber having a small numerical aperture and a small core diameter wasused in order that almost all of the radiated light passes through aU-shaped detection cell. Light passed through a single mode opticalfiber diverges conically at a predetermined angle. The diameter (w) oflight from the single mode optical fiber is calculated by:

w=d×(0.65+1.619/V ¹⁵+2.879/V ⁶)  (2)

[0017] where d is the diameter of the optical fiber, and V=d×π×NA/λwhere NA is the numerical aperture of the optical fiber, and X is thewavelength of radiated light.

[0018] With this type of a conventional absorption detection system in alab-on-a-chip, an optical fiber having a core diameter of 3 μm, acladding diameter of 125 μm, and an NA of 0.1 is used, and 488-nm lightis radiated from an argon ion laser. The diameter of light from thisoptical fiber, which can be calculated by formula (2) above, is 3.93 μm.Here, the divergence angle (θ) of light is calculated by:

θ=arcsin(NA/n)  (3)

[0019] where n is the refractive index of a medium through which lighttransmits (n=1.33 for water, n=1.52 for glass). The diameter (w′) ofdivergent light at a distance (X) from the medium is calculated by:

w′=w+2X tanθ  (4)

[0020] As an example, assuming that light from a single mode opticalfiber passes through a detection cell filled with water and having alength of 150 μm, the divergent light from the detection cell, which canbe calculated by the formulae above, has a diameter of about 27 μm. Ifthe detection cell has a length of 500 μm, the divergent light from thedetection cell has a diameter of about 80 μm. Therefore, when designinga U-shaped detection cell having a depth of 25 μm and a width of 50 μmfor a lab-on-a-chip using a single mode optical fiber, the length of theU-shaped detection cell is determined to be no larger than 150 μm toallow almost all the light radiated through the single mode opticalfiber to pass through the U-shaped detection cell. As a result, theconventional absorbance detection system in a lab-on-a-chip showed anincrease in detection sensitivity by only 3-4 times of that of adetection method in an non-extended detection cell.

[0021] In addition, because the conventional absorbance detection systemin a lab-on-a-chip is fabricated in glass, it is difficult to fabricate,is time consuming and has low reproducibility. Also, the detection cell(channel) of the absorbance detection system has a circularcross-section and thus generates a serious light scattering problem.

SUMMARY OF THE INVENTION

[0022] To solve the above problems, it is an object of the presentinvention to provide a high-efficiency, high-sensitivity absorbancedetection system in a lab-on-a-chip in which detection cells with longoptical pathlengths for higher detection sensitivity, micro-lenses forcollimating light into the detection cells, and slits for preventingscattered light from entering the detectors are fabricated andintegrated in the lab-on-a-chip.

[0023] Another object of the present invention is to provide anabsorbance detection system in a plastic lab-on-a-chip capable ofsolving the problems of conventional absorbance detection systems inlab-on-a-chips and produces a 10 times greater detection sensitivity.

[0024] Still another object of the present invention is to provide anabsorbance detection system in a lab-on-a-chip in which a liquid- orsolid-state waveguides or micro-light sources (lamp or laser) as well asoptical fibers can be used for light radiation.

[0025] Yet still another object of the present invention is to providean absorbance detection system in a lab-on-a-chip in which an additionalcollimator, including a micro-lenses and slits, are arranged close todetection cells so that effective absorbance detection can be achievedusing the detection cells with optical pathlengths of 50 μm or longer.

[0026] The present invention also aims to provide an absorbancedetection system in a lab-on-a-chip in which collimators, includingmicro-lenses for collimating divergent light from optical fibers andtransmitting the collimated light into detection cells and slits forpreventing light that does not pass through the detection cells orscattered light from entering detectors, are built-in so that detectionsensitivity can be improved markedly with the increased opticalpathlengths of the detection cells.

[0027] In achieving the objects of the present invention, thefabrication of a lab-on-a-chip is carried out to build an absorbancedetection system comprising: a detection cells having opticalpathlengths of 50 μm-5 mm; lens structures for collimating andtransmitting light to the detection cells; and slit structures forpreventing scattered light from entering detectors.

[0028] In one embodiment, the lens structures and the slit structuresmay form collimators to transmit collimated light to the detectioncells.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above objects and advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

[0030]FIGS. 1A and 1B are plan views of preferred embodiments of anabsorbance detection system in a lab-on-a-chip according to the presentinvention;

[0031]FIG. 2 is a magnified perspective view showing each layer of theabsorbance detection system according to the present invention;

[0032]FIG. 3 illustrates the fabrication process of top and bottomplates for the absorbance detection system according to the presentinvention;

[0033]FIG. 4 illustrates the fabrication process of a membrane plate forthe absorbance detection system according to the present invention andthe bonding of the membrane plate to the top and bottom plates;

[0034]FIG. 5 shows an experimental setup for absorbance detection in alab-on-a-chip according to the present invention;

[0035]FIG. 6 is a photograph of an absorbance detection system without acollimator near the detection cell according to the present invention;

[0036]FIG. 7 is a photograph showing light passing through the detectioncell of the absorbance detection system shown in FIG. 6;

[0037]FIG. 8 is a photograph showing collimated light passing throughthe detection cell in an absorbance detection system with a collimatoraccording to the present invention;

[0038]FIG. 9 is a sectional view showing the detection cell surroundedby slits in the absorbance detection system with a collimator accordingto the present invention;

[0039]FIG. 10 is a graph comparatively showing absorption efficiencybetween detection cells of the absorbance detection systems according tothe present invention; and

[0040]FIG. 11 is a graph comparatively showing detection sensitivitybetween detection cells of the absorbance detection systems according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0041] Preferred embodiments of an absorbance detection system for alab-on-a-chip according to the present invention will be described indetail with reference to the appended drawings. Description of the priorart or the unnecessary structure of the present invention that makes thesubject matter obscure will be omitted. The terms used in the followingdescription are defined on the basis of functions, and thus it will beappreciated by those skilled in the art that the terms may be changedappropriately based on users' or operators' intentions and practiceswithout departing from the meaning intended in the followingdescription.

[0042]FIGS. 1A and 1B show preferred embodiments of an absorbancedetection system in a “lab-on-a-chip” according to the presentinvention. In particular, FIG. 1A shows a high-performance absorbancedetection system 100 built in a lab-on-a-chip having a collimator, andFIG. 1B shows an absorbance detection system 100′ built in alab-on-a-chip that does not have a collimator. In this specification,the collimator is defined as comprising a lens 25 and slits (channels)19 and collimates light for an absorbance detection cell (hereinafter,“detection cell”) 1. The collimator is not designated by a particularreference numeral in the following description.

[0043] In FIGS. 1A and 1B, reference numeral 13 denotes a samplereservoir, reference numeral 14 denotes a buffer reservoir, referencenumeral 15 denotes a sample waste reservoir, reference numeral 16denotes a buffer waste reservoir, and reference numeral 17 denotes anink reservoir for containing ink used to fill the slits 19. Referencenumeral 18 denotes a separation channel having a width of 5 μm to 1 mm.The width of the separation channel 18 is determined depending on theapplication field.

[0044] In the absorbance detection systems 100 and 100′ in thelab-on-a-chip shown in FIGS. 1A and 1B, both the detection cells 1 havethe same optical pathlength, of 50 μm to 5 mm, which is determineddepending on the application field. The detection cells 1 are connectedto the respective separation channels 18.

[0045] In FIG. 1A, an element denoted by reference numeral 25 andlocated near the end of a source fiber channel 20 is a micro-lens havinga diameter of 5 μm to 1 mm, which collimates light. A convex lens, aconvex-concave compound lens, or a gradient-index(GRIN) lens can be usedas the micro-lens 25.

[0046] In designating the constituent elements of the absorbancedetection system according to the present invention, the terms including“channel”, for example, “optical fiber channel” or “slit channel”,define constructions for corresponding functional elements, i.e., for“optical fiber” or “slit”. Therefore, the terms “optical fiber channel”and “optical fiber” or “slit channel” and “slit” can be used as havingthe same meaning through the specification.

[0047] Reference numeral 19 denotes a slit (channel) for preventingscattered light from entering a detection cell (not shown) and is filledwith a light absorbing material such as black ink for use. Referencenumeral 20 denotes a source fiber channel having a width of 5 μm to 1mm. Reference numeral 21 denotes a collection fiber channel, whichreceives light from the detection cell 1 and transmits the same to adetector (see FIG. 5). In the source and collection fiber channels 20and 21, the outer end of each channel is formed to be wider than theinner end thereof for easy insertion of an optical fiber into thechannel.

[0048] Fabrication of the absorbance detection systems 100 and 100′ ineach of the lab-on-a-chip shown in FIGS. 1A and 1B will be describedwith reference to FIGS. 2 through 4.

[0049] An absorbance detection system in a lab-on-a-chip in which fiberchannels and 3-dimensional slits 19 are arranged in front of and behinda detection cell 1 is fabricated as a three-layered structure usingthree photomasks. FIG. 2 is a magnified perspective view of theabsorbance detection system 100 in a lab-on-a-chip.

[0050] Referring to FIG. 2, the source fiber channel 20, the collectionfiber channel 21, and the slit channel 19, each having a depth of 5 μmto 1 mm, are formed in an top plate 30. A membrane plate 40 as thin as 5μm to 1 mm has the separation channel 18, the detection cell 1, theoptical fiber channel 20, and the slit channel 19, which are formedthrough the membrane plate 40. In the membrane plate 40, the microlens25, preferably, a micro-planoconvex lens, is formed on one end of thesource fiber channel 22.

[0051] As in the top plate 30, the source fiber channel 20, thecollection fiber channel 21, and the slit channel 19, each having adepth of 5 μm-1 mm, are formed in a bottom plate 50. Here, the width ofthe slit channels 19 in the top plate 30 and the bottom plate 50 areformed to be 5-10 times wider than that of the detection cell 1. In themembrane plate 40, each of the slit channels 19 are divided with aseparation gap corresponding to the width of the detection cell 1.

[0052] Preferably, the top, membrane, and bottom plates 30, 40, and 50for the lab-on-a-chip are formed in plastics includingpolydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA),polycarbonate (PC), polyethylene (PE), polypropylene (PP), andpolystyrene (PS). In forming such micro-channels and structures in thetop, membrane, and bottom plates 30, 40, and 50, a molding techniqueusing a template shaping a molten plastic by hardening, a hot embossingtechnique in which a planar substrate is hot pressed with a template, orother processing techniques, such as casting, mechanical machining, andlaser machining, using a mechanical tool or a light or heat source canbe applied.

[0053] As an example, a method for manufacturing the top, membrane, andbottom plates 30, 40, and 50 using PDMS by molding will be describedbelow.

[0054] The top and bottom plates 30 and 50 with channels are fabricatedas follows. Referring to FIG. 3, a negative-type photoresist 101, SU-8,is coated on a silicon wafer 102 to a thickness of 5 μm to 1 mm, and aphotomask 103 having a pattern is placed on the silicon wafer 102 andexposed to ultraviolet (UV) light. The UV-exposed negative-typephotoresist 101 is developed in a developer solution to form a positivemold 104 with a pattern having a height of 40 to 250 μm. PDMS 105 isspread over the positive mold 104 and peeled off after cross-linking,resulting in the top plate 30 or bottom plate 50 having a negativepattern for a lap-on-a-chip.

[0055] Processes for fabricating the membrane plate 40 for theabsorbance detection system 100 and bonding the membrane plate 40 to thetop and bottom plates 30 and 50 will be described with reference to FIG.4.

[0056] As described with reference to FIG. 3, a negative-typephotoresist 101 is coated on a silicon wafer 102 to a thickness of 5 μmto 1 mm, and a photomask 103 a having a desired pattern specified forthe membrane plate 40 is placed on the silicon wafer 102 and exposed toUV light. The UV-exposed negative-type photoresist 101 is developed toform a positive mold 104 a with a pattern having a height of 5 μm-1 mm.PDMS 105 a is spread over the positive mold 104 a and then pressed witha flat PDMS plate 107 to transfer the positive pattern of the positivemold 104 on the spread PDMS 105 a by cross-linking so that the membraneplate 40 is formed. The PDMS plate 107 is oxidized by corona dischargeusing a Tesla coil (not shown) and silanized so that the membrane plate40 formed of PDMS can be separated easily from the PDMS plate 107.

[0057] After separating the membrane plate 40 and the PDMS plate 107from the silicon wafer 102, on which the positive mold 104 a is formed,the membrane plate 40 and the top plate 30 are subjected to surfacetreatment using the Tesla coil and bonded to each other after patternalignment. It will be appreciated by those skilled in the art thatsurface treatment performed on PDMS plates using a Tesla coil beforebonding can be applied to glass plates or silicon wafers.

[0058] After separating the PDMS plate 107 from the membrane plate 40,reservoir holes for the reservoirs 13 through 17 shown in FIG. 1 arepunched in the assembly of the membrane plate 20 and the top plate 30.Next, the other surface of the membrane plate 40 and the bottom plate 50are subjected to surface treatment, and the bottom plate 50 is bonded tothe membrane plate 40 after alignment, resulting in the absorbancedetection system 100 in a lab-on-a-chip.

[0059] Referring to FIG. 5, 200 μL pipette tips cut to appropriate sizesare inserted into the reservoir holes in the absorbance detection system100. A source fiber 209 having a core diameter of 1 μm to 1 mm and acladding diameter of 2 μm to 2 mm is inserted into the source fiberchannel 20 (see FIG. 1). A collection fiber 210 having a core diameterof 1 μm to 1 mm and a cladding diameter of 2 μm to 2 mm is inserted intothe collection fiber channel 21 (see FIG. 1). When a collimator isarranged near the detection cell 1, as shown in FIG. 1A, the sourcefiber 209 must be positioned at the focal length of themicro-planoconvex lens 25 (see FIG. 2) to emit collimated light to thedetection cell 1. Finally, a material capable of adsorbing scatteredlight, for example, black ink, is put into the reservoir 17 (see FIG. 1)and fills the slit channels 19.

[0060] The structure and operation of a separation and absorbancedetection system using an absorbance detection system in a lab-on-a-chipaccording to the present invention will be described with reference FIG.5.

[0061] A preferred embodiment of a separation and absorbance detectionsystem for the lab-on-a-chip 100(100′) is shown in FIG. 5. Samplemigration, injection, and separation in the lab-on-a-chip 100(100′) areachieved by electroosmotic flow. To this end, a high-voltage powersupply 201, a voltage divider 202, and a high-voltage relay 203 areused. Sample injection is performed by a gate injection method. In thiscase, voltages applied to the sample reservoir 13, the buffer reservoir14, and the sample waste reservoir 15 are adjusted by the voltagedivider 202. Before injection, a 0.1 to 10 kV is applied to the samplereservoir 13, a voltage 0.3 to 0.9 times the voltage provided to thesample reservoir 13 is applied to the buffer reservoir 14, and a voltage0 to 0.9 times the voltage provided to the sample reservoir 13 isapplied to the sample waste reservoir 15. The buffer waste reservoir 16is connected to ground.

[0062] For sample injection, the voltage applied to the buffer reservoir14 is floated for a predetermined time period, e.g. 0.01 to 100 seconds,by the high-voltage relay 203 and then reapplied. The injected sample isseparated while passing through the separation channel 18 and thendetected by absorbance detection using a UV lamp, such as deuterium,mercury, tungsten, or xenon lamp, or a kind of laser along with anoptical fiber. In the present embodiment, an argon ion laser 208emitting a wavelength of 488 nm is used as a light source. Light fromthe argon ion laser 208 is transmitted to the detection cell 1 throughthe source fiber 109, and the light from the detection cell 1 istransmitted to the detector 211 through the collection fiber 210. Thedetector (photo diode or photo multiplier tube (PMT)) 211 measures theintensity of the collected light. A computer 212 controls thehigh-voltage power supply 201 and the high-voltage relay 203 and recordsand stores the signals output from the detector 211.

[0063] A light path in the detection cells in different absorbancedetection systems will be comparatively described with reference toFIGS. 6 through 9. In FIGS. 6 through 9, reference numeral {circle over(1)} denotes a detection cell, reference numeral {circle over (2)}denotes a source fiber, reference numeral {circle over (2)}′ denotes acollection fiber, reference numeral {circle over (3)} denotes aplanoconvex lens, and reference numeral {circle over (4)} denotes aslit.

[0064] Light propagation in the detection cells after being emittedthrough the optical fibers of the absorbance detection systems 100 and100′ fabricated as described with reference to FIGS. 2 through 4 wascaptured using a charge-coupled device (CCD) camera. The results areshown in FIGS. 6 and 9 for comparison. FIG. 6 shows the absorbancedetection system 100′ without a collimator (hereinafter, “non-collimatedsystem”) near the detection cell 1 before light emission. The sourcefiber {circle over (2)} having a core diameter of 3 μm is insertedthrough the source fiber channel 20 (see FIG. 1B or 2), and thecollection fiber {circle over (3)} having a core diameter of 50 μm isinserted through the collection fiber channel 21 (see FIG. 1B or 2).FIG. 7 shows the non-collimated system 100′ without a collimator nearthe detection cell 1 after light emission in which the detection cell 1is filled with a fluorescent compound (fluorescein) over extendedinjection time and is irradiated by light from an argon ion laserthrough the source fiber 2. As shown in FIG. 7, in the non-collimatedsystem 100′, light is diverged in the detection cell 1. Evidently, it ishard for all of the light to entirely pass through the detection cell 1.

[0065]FIG. 8 shows the absorbance detection system 100′ with acollimator (hereinafter “collimated system”) near the detection cell 1in which the detection cell 1 is filled with fluorescein and collimatedlight from the microlens passes through the detection cell 1. Here, thesource fiber 2 is arranged at the focal length of the micro-convex lens3 to obtain collimated light. The slits 4 filled with black ink act asbarriers to absorb scattered light. FIG. 9 illustrates a section of thedetection cell 1 surrounded by the 3-dimensional slits 4.

[0066] The absorption efficiency of the detection cells in theabsorption detection systems according to the present invention will bedescribed with reference to FIG. 10.

[0067] To determine the absorption efficiency of the collimated andnon-collimated detection cells, each of the detection cells is filledwith a 5-1500 ppm New coccine solution and irradiated through opticalfibers. Next, the intensity of the light passed through each of thedetection cells is determined using a photodetector.

[0068] Absorbance for a sample solution is calculated by:

A=log(I ₀ /I)

[0069] where I₀ and I denote the intensity of a signal from a PMT for adetection cell before and after being filled with the solution,respectively. The larger the concentration of solution, the smaller thevalue of I and the larger the absorbance. According to Beer's law,absorbance of a sample is proportional to its concentration, and thusabsorbance of the sample varies linearly with respect to itsconcentration. When a portion of the incident light improperly passesthe detection cell or scattered light enters the detection cell, therange of concentration in which the linearity is satisfied is reducedwith a decreased slope.

[0070]FIG. 10 is a graph comparatively showing absorbance with respectto concentration for a collimated detection cell 1 and a non-collimateddetection cell 1′. As shown in FIG. 10, the collimated detection cell 1shows linear absorbance through the entire concentration range, i.e.,between 0 and 1500 ppm, whereas the non-collimated detection cell 1′shows linear absorbance up to a concentration of 500 ppm. The absorptionefficiency of the collimated detection cell 1 is better than that of thenon-collimated detection cell 1′. Also, a reduction in efficiency due toscattered light was not observed in the collimated detection cell 1.

[0071] Detection sensitivities of the detection cells according to thepresent invention were compared. The results are shown in FIG. 11. FIG.11 shows the results of separation of a sample when a non-extendeddetection cell 11′ having an optical pathlength of 50 μm and acollimated detection cell 11 having an optical pathlength of 500 μm areused. The sample used was a mixture of 10 μM of fluoroscein, 50 ppm ofOrange II, and 50 ppm of New coccine and was injected for 0.5 seconds.

[0072] As shown in FIG. 11, the collimated detection cell 11 shows about10 times higher sensitivity for each peak than the 50-μm detection cell11′. Also, the collimated detection cell 11 shows an improved detectionlimit at 1 to 3 ppm, compared to the detection cell 11′ which has adetection limit of 10 to 30 ppm. Separation efficiency is not reducedfor the collimated detection cell 11 having such an extended opticalpathlength.

[0073] As described above, the absorbance detection system in alab-on-a-chip according to the present invention can be applied for thedetection of a variety of samples. The absorbance detection system in alab-on-a-chip according to the present invention does not need labelingof the samples which saves time and costs compared to a conventionaldetection method. Therefore, the absorbance detection system in alab-on-a-chip according to the present invention has high-efficiency andhigh-sensitivity.

[0074] The absorbance detection system in a lab-on-a-chip according tothe present invention can be used with wider applications to a varietyof samples in diverse research fields, for example, the drug screeningfield dealing with simultaneous synthesis and identification of a numberof substances based on combinatorial chemistry, the life sciences fieldhandling trace bioactive materials such as enzymes, proteins, and aminoacids, and the environmental research field which needs rapid fieldmonitoring of contaminants.

[0075] The absorbance detection system in a lab-on-a-chip according tothe present invention can be used effectively in detecting tracecompounds with a much higher sensitivity than a conventional absorbancedetection system.

[0076] While this invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. An absorbance detection system in alab-on-a-chip, comprising: detection cells having optical pathlengths of50 μm to 5 mm; lens structures with lenses for collimating light in thedetection cells; and slit structures for preventing scattered light fromentering detectors, the detection cells, the lens structures, and theslit structures being integrated in the lab-on-a-chip.
 2. The absorbancedetection system of claim 1, wherein the lenses for collimating light inthe detection cells are convex lenses, convex-concave compound lenses,and/or gradient-index(GRIN) lenses.
 3. The absorbance detection systemof claim 1, wherein the slit structures are filled with materialscapable of absorbing scattered light.
 4. The absorbance detection systemof claim 3, wherein the materials capable of absorbing scattered lightare black ink, organic dye solutions, and/or inorganic dye solutions. 5.The absorbance detection system of claim 1, wherein the detection cells,the lens structures, and the slit structures are formed from the samematerial as that of the lab-on-a-chip.
 6. The absorbance detectionsystem of claim 1, wherein the detection cells, the lens structures, andthe slit structures are formed from different materials to that of thelab-on-a-chip and then integrated into the lab-on-a-chip.
 7. Theabsorbance detection system of claim 1, wherein the lab-on-a-chip isproduced from at least one plastic material selected from the groupconsisting of polydimethylsiloxane (PDMS), polymethylmethacrylate(PMMA), polycarbonate (PC), polyethylene (PE), polypropylene (PP), andpolystyrene (PS).
 8. The absorbance detection system of any of claims 1through 6, wherein optical fibers or optical waveguides are used forguiding light from light sources to the lens structures and/or from thedetection cells to light detectors.
 9. The absorbance detection systemof any of claims 1 through 6, wherein micro-light sources selected fromthe group consisting of lamps, light emitting diodes and lasers are usedas light sources for absorbance detection.
 10. The absorbance detectionsystem of claim 1, wherein the lens structures and the slit structurescomprise collimators to form collimated light in the detection cells.11. The absorbance detection system of claim 1, wherein the absorbancedetection system in the lab-on-a-chip is fabricated by methods selectedfrom the group consisting of molding, embossing, casting, mechanicalmachining, and laser machining.